1. Field of the invention:
This invention relates to a system for controlling the pressure of hydraulic operating fluid supplied into hydraulic actuators for speed-change clutch-brakes of a multi-stage power shift transmission.
2. Description of the prior art:
The control system of the type specified comprises, in general, a plurality of hydraulic speed-change actuators such as, for example, a plurality of speed-change clutch-brake cylinders; a group of directional control valves adapted to supply hydraulic fluid under pressure selectively into the clutch-brake cylinders; hydraulic fluid circuits for supplying the hydraulic fluid under pressure delivered from a pressurized fluid supply source through the directional control valves into the plurality of speed-change clutch-brake cylinders, and a group of modulating valves adapted to modulate the pressure of hydraulic operating fluid in the hydraulic fluid circuits.
The hydraulic operating fluid for actuating the above-mentioned clutch-brakes is supplied at a predetermined pressure through a hydraulic fluid circuit into the speed-change clutch-brake cylinder or cylinders selected by the operation of the directional control valves, and then increased in hydraulic pressure therein so as to effect the clutch-brake actuating function. Needless to say, it is necessary for safe and reliable speed-change operation that the speed-change operation is made uniformly through the whole speed stages. However, there occurs irregularity of the speed-change operation because of the resistance in the hydraulic fluid circuits or passages, leakage of hydraulic operating fluid and the difference in volume among clutch-brake cylinders, etc. Particularly, in the hydraulic power shift transmission gear including a rotary type clutch-brake speed change mechanism, it takes a longer time to fill up the mechanism with hydraulic operating fluid as compared with that required in the case of controls of speed-change clutch-brake pressure in a planetary gear type speed reduction system. For this reason, a delay of timing will occur between disengagement of the clutch-brake before the speed-change operation and engagement of the clutch-brake selected by the operator. As a result, a period during which the transmission of the power is cut off is prolonged thus causing a speed drop of the vehicle and shock at the time of speed changes.
To solve such problems, there have so far been taken a measure wherein the volume of hydraulic operating fluid to be discharged from the speed-change clutch-brake cylinder is restricted so as to delay the release of the clutch-brake to enable the power transmission condition to be maintained during the period between the disengagement of the speed-change clutch-brake and the commencement of engagement of the clutch-brake selected by the operator, or another measure as disclosed in the Japanese Patent Publication No. SHO 57-16259 wherein engagement and disengagement of the speed-change clutch-brakes are controlled electrically so as to compensate the hydraulic fluid pressure drop which occurs when the clutch-brake is released with the hydraulic fluid pressure rise at the time when the clutch-brake is engaged thereby preventing the possible interruption of power transmission.
The above-mentioned known measures are effective technical approaches. However, the former measure wherein the discharge of the pressurized hydraulic fluid actuating the clutch-brake is restricted is disadvantageous in that it is difficult to define the size of the restrictor and adjust combination of a plurality of speed-change clutch-brakes, and therefore a stable timing of releasing the clutch-brake cannot be obtained. Whilst, the latter measure wherein the clutch-brake release timing is allowed to overlap the timing of engaging the selected clutch-brake to achieve the hydraulic fluid pressure control is disadvantageous in that it is complicated in structure and expensive in construction cost.